The present invention relates to forming an article having a surface which is exposed to a reactive metal during casting of the reactive metal.
During the casting of reactive metals, such as nickel-chrome superalloys, titanium, zirconium or alloys containing substantial amounts of tungsten, hafnium, carbon, nickel, etc., a reaction tends to occur between the alloy and surfaces exposed to the molten metal. The reaction between one or more of the exposed surfaces and the metal may be so severe as to result in defects which render a casting unsuitable for its intended purpose. These defects may be the result of a chemical interaction between the metal and the surface or due to migration of elements from the metal to the exposed surface. The articles having surfaces which are exposed to the molten metal include ladle liners, crucibles, molds, filters and cores.
During the casting of single crystal, polycrystalline, and directionally solidified blades and vanes, the molten metal is exposed to surfaces on a core disposed in a mold. During casting, reactions tend to occur between the ceramic core material and the superalloy metal. These reactions can be particularly severe when the metal is a nickel-chrome superalloy containing yttrium, such as GE N5 and N6, and PWA 1487.
During the casting of nickel-chrome superalloys containing yttrium, silica cores cannot be used due to their reactivity and inability to withstand the 1,500.degree.-1,600.degree. C. casting temperatures. Although alumina cores are dimensionally stable at these temperatures, they tend to react with components of the superalloy and lead to castings with poor air flow characteristics and poor structural integrity. Thus, when a nickel-chrome superalloy containing yttrium is cast with an alumina core, reactions occur which tend to promote migration of the yttrium from the molten metal to the core body and forms a reaction layer between the core and the metal upon solidification.